Motor vehicles are one of the major contributors to air pollution both in the United States and abroad. Thus, it is desirable to measure exhaust emissions from automobiles under actual operating conditions in order to identify excessively polluting vehicles and either have them repaired or have them removed from the road. Apparatus for detecting and measuring relative concentrations of pollutants such as HC, CO, and CO2 in the exhaust emissions from automobiles under operating conditions have been developed. Examples of such apparatus are shown in U.S. Pat. Nos. 5,343,043 (Johnson) and 5,210,702 (Bishop, et al.), which are incorporated by reference herein.
In order to properly analyze the exhaust emissions data obtained, the speed and/or acceleration of the vehicle at the time the exhaust emissions information was obtained is very useful. More specifically, the emissions test results can be influenced by the specific mode of operation, for example, acceleration or decceleration, of the vehicle under the test. Determining the mode of operation must be performed in a very short distance of travel, so as to capture instantaneously the specific accelerator position. This in turn supplies information about fuel enrichment and fuel deprivation of the engine at the specific time of the emissions test.
Devices have been developed in the past to determine the speed of vehicles. One such device is the radar detector used by law enforcement agencies. Although such radar devices are quite useful, radar waves can reflect off several points of the same vehicle, resulting in obtaining different speeds for the same vehicle traveling at a constant velocity. Another device for determining the speed of motor vehicles is the mechanical switch. Mechanical switches, however, have a great variance in response time. Also, cumbersome methods are needed, such as taping the sensors down to the highway, to keep the switches at a fixed length apart. Still another device is the use of modulated infrared laser beams. The modulated infrared laser beam has a set time for the beam to be on and off. This time translates into time steps that can cause error in the detection of the laser beam and, thus, accurate and reliable results may not be obtained.